Actively crosslinked microtubule networks: mechanics, dynamics and filament sliding

Cellular components such as cytoskeletal filaments and motors are the essential
constituents of a new class of materials: so called active fluids. While much
progress has been made in understanding these systems using experiments and
phenomenological theories, deriving rigorous theoretical description from
microscopic considerations remains a challenge. We present experiments and theory
on a system of stabilized microtubules driven by the molecular motor protein
XCTK2. Through photobleaching experiments, we demonstrate that in this system
microtubules are aligned along the long direction of the system and travel
through the gel at a velocity independent of the local average polarity. We show
that this result is most naturally understood in the frameworks of an active gel
theory that goes beyond pairwise microtubule interactions and treats the gel as
highly cross-linked. Our theory bridges the length scales from the microscopic
mechanical behavior of motor-filament interactions to the large scale behavior
of the active gel and generalizes to describe different kinds of cytoskeletal
assemblies.